Bhagavad Gita · 18.59

यदहङ्कारमाश्रित्य न योत्स्य इति मन्यसे।मिथ्यैष व्यवसायस्ते प्रकृतिस्त्वां नियोक्ष्यति॥

yad ahaṅkāram āśhritya na yotsya iti manyase mithyaiṣha vyavasāyas te prakṛitis tvāṁ niyokṣhyati

If, filled with ego, you think 'I will not fight,' your resolve is vain. Your own nature will compel you.

— Bhagavad Gita 18.59

Source: Sacred-Texts.com (Sanskrit) + Bhagavad-Gita.io (translations)